The present invention generally relates to pressure control in a cabin of an aircraft and, more particularly, provision of pressure relief which may be required maintain cabin pressure within safe limits.
In those aircraft that may operate at altitudes greater than about 8000 feet, passenger compartments or cabins are provided with controlled pressurization. Typically, pressurization may be controlled to maintain a cabin pressure that corresponds to that which would exist at about 6000 to 8000 feet above sea level. When an aircraft reaches an altitude in excess of 6000 to 8000 feet, a positive pressure differential may develop between an interior of the cabin and an exterior of the cabin. In the case of an aircraft at 45,000 feet, for example, this pressure differential may be as large as 10 pounds per square inch differential (PSID). This situation where the cabin interior pressure is greater than the exterior, ambient, pressure is defined as a positive differential pressure. It is also possible to operate the aircraft whereby the exterior pressure can exceed the interior pressure. This situation is defined as a negative differential pressure condition. Aircraft with pressurized cabins are designed with sufficient fuselage structural strength to withstand forces that may be generated by these anticipated pressure differentials. However, if an aircraft fuselage is exposed to a pressure differential in excess of the anticipated differential, there is a risk of overstressing the fuselage and, in an extreme case; there is even a risk of structural failure of the fuselage.
Fuselage structural strength is usually designed to withstand positive differential pressure magnitudes between 6 to 10 PSID, depending on the flight altitudes the airplane is designed to achieve. However, the fuselage structural strength is usually only designed to withstand negative differential pressures up to magnitudes around −0.25 PSID. Excess positive or negative pressure differential may occur if there is a malfunction or failure of a cabin pressure control system. In order to mitigate such a risk, all modern day aircraft are provided with an independent pressure relief valve which may operate to rapidly limit and regulate an excess pressure differential that may arise as a result of a failure of the cabin pressure control system. Additionally, certification regulations require pressure relief valves that automatically limit the positive and negative pressure between the inside and the outside of the fuselage structure. Some aircraft have pressure relief valves that perform both the positive and negative pressure relief function using the same valve. However, some aircraft utilize two different types of valve designs to perform the positive versus the negative pressure relief functions. This use of two different valve designs is often the case for aircraft types that are large in size, as the negative pressure relief valve size can become significantly larger than the positive pressure relief valve size, causing the weight of a combined positive and negative pressure relief valve to be too great for effective use on an airplane.
A typical prior art pressure relief valve may be set to operate when cabin-to-ambient pressure differential (hereinafter ΔP) exceeds a single predetermined limit. The predetermined limit may be established for a particular aircraft design to meet regulatory safety requirements for an expected maximum operating altitude. In order for the relief valve to meet its required protective functionality, the valve must be provided with a sensing system that is independent from the aircraft cabin pressure control system so that a failure of the cabin pressure control system will not result in a failure of the relief valve. Also, a typical prior art positive pressure relief valve may a have sensing system and a valve actuating system that is all pneumatic. If a dedicated negative pressure relief valve is used on the airplane, its design may rely on a mechanical spring to shut a valve plate until the pressure difference across the valve plate exceeds the spring force. Then the negative pressure relief valve opens to ingress air to the interior of the airplane.
A typical prior art pneumatically controlled pressure relief valve utilizes the difference between the cabin and the atmosphere air to provide motive force for actuation. When the airplane is on the ground during normal taxi and gate operations, the ΔP across the fuselage is inadequate for the pressure relief valve to actuate from the closed position. Because an independent pressure relief valve is required to meet regulatory safety requirements, and because the pressure relief valve is only operated for its function after there is a failure of the automated outflow control system, the health status of pressure relief valve functionality must be periodically verified during maintenance checks of the airplane to verify that there is not a latent defect. Maintenance checks for pneumatically actuated valves must be performed either during a flight test, or by pressurizing the airplane on the ground, or by actuating the pressure relief valve using special pneumatic test equipment. A pneumatically actuated negative pressure relief valve is only ever actuated when a negative overpressure is experienced. Thus this valve type may also have a latent defect that would only be discovered during a maintenance check.
Another aspect of safely controlling the cabin pressure control function is that the cabin altitude must always be limited to ensure passenger safety. Aircraft certification regulations require that the cabin altitude be limited to less than 15,000 ft for most failure conditions. Thus, it is possible for a pressure relief valve, if it were to erroneously open fully during flight, to allow a cabin decompression beyond 15,000 ft in some circumstances. Existing art pneumatic pressure relief valves have some design techniques that prevent their opening during most failure conditions. However, there are still other pressure relief valve failure modes that would allow a complete opening of the pressure relief valve in flight to allow a decompression beyond 15,000 ft.
Some commercial aircraft may be operated in high-cycle modes. In other words, some aircraft may be employed in relatively short flights that reach only limited altitudes such as 35,000 feet. These short flights operations may result in a relatively high number of take-offs and landings along with a high number of cabin pressurization cycles. Increased longevity of a high-cycle aircraft could be achieved if potential ΔP were not allowed to rise as high as that which might develop at a high altitude such as 45,000 feet. But, in the event of a failure of the automated outflow control system, a relief valve set for operation at a 45,000 ft. operating altitude may potentially permit ΔP to rise to 10 PSID, even if an aircraft operates only at 35,000 feet. Consequently, regulatory safety requirements for aircraft structure mandate that each cycle of aircraft operation is deemed to have produced fuselage stress at a level that would have occurred during operation. These regulations provide limits on the number of cabin pressurization cycles that may be allowed between maintenance intervals or before retirement of an aircraft. The regulatory requirements take into account the reality that the aircraft may have operated only at a low altitude and experienced cabin pressurization substantially lower than the set point of the relief valve, but they also consider that the automated outflow control system may have failed and that some of the operating flight cycles may have experienced a ΔP equivalent to the pressure relief valve set for operation at 45,000 ft altitude. Therefore the regulatory requirements may reduce the quantity of cycles that a high-cycle aircraft may achieve.
It must be noted that high-cycle aircraft may occasionally operate at high altitudes such as 45,000 feet. Thus, a pressure relief valve with a set point for airplane operation only to 35,000 ft would prevent normal automated outflow valve control operation up to 45,000 ft. In those circumstances, cabin pressurization may indeed rise as high as 10 PSID. Consequently, a relief valve with a single set point must be set to operate at a pressure no less than 10 PSID. Otherwise, if the relief valve were set to operate at a lower differential pressure, proper cabin pressurization could not be attained for the occasional high altitude flight.
Some commercial aircraft are designed to optimize structural weight by limiting combinations of pressure loads and fuselage loads caused by in-flight aerodynamic maneuvering loads. It is possible that due to increased air density at flight altitudes less than, for example, 10,000 ft, that aerodynamic maneuvering loads may be higher than those loads developed by similar maneuvers at much higher altitudes. Thus, when considering airplane structural weight, it would be advantageous to reduce the pressure loading due to ΔP for altitudes less than 10,000 ft when the aerodynamically induced maneuver loads are at their greatest.
As can be seen, it would be desirable to provide a cabin pressure relief system that takes into account the actual operating altitude of an aircraft and has have a ΔP limiting and regulating function that is independent from the rest of the cabin pressure outflow control system. Additionally, it would be desirable to construct pressure relief valves with the means to automatically perform periodic functional checks without maintenance crew interaction. It would also be advantageous to combine the positive and negative pressure relief valve functions into a single pressure relief valve. Also, it would be an advantage that a pressure relief valve has means to prevent it from opening and causing a decompression beyond 15,000 ft.